NEW YORK (Reuters Health) - Colon and lung cancer patients who regularly took low-dose aspirin before their diagnosis tended to have less advanced tumors, in a new study.

Scientists already knew that aspirin was tied to a decreased risk of death for people with colon cancer, said senior author Yudi Pawitan.

"We showed evidence that it is also beneficial for lung cancer, and has both early and late protective effects," Pawitan, of the department of medical epidemiology and biostatistics at the Karolinska Institutet in Stockholm, Sweden, told Reuters Health.

However, the finding doesn't mean everyone should be taking aspirin to ward off advanced cancer, researchers said.

Pawitan and his coauthors analyzed data from Swedish cancer and prescription drug registries that included 80,000 patients with colorectal, lung, prostate or breast cancer.

One in four people with colorectal, lung or prostate cancer had regularly taken low-dose aspirin before being diagnosed - typically one 75-milligram tablet per day - compared to about one in seven breast cancer patients.

The researchers found 20 to 40 percent fewer colon, lung and breast cancer patients who had taken aspirin had tumors that had spread to other areas of the body than those who had not taken aspirin.

For example, 19 percent of regular aspirin users with colon cancer had metastatic disease, compared to close to 25 percent of non-users.

Tumors on average were smaller and less advanced among aspirin users with colon and lung cancer, but not those with breast or prostate cancer, according to results published in the British Journal of Cancer.

"The fact that they did not find a similar result for breast and prostate cancer does not exclude the possibility that aspirin may work at a different point in the cancer process for those cancers," said Dr. Michelle Holmes, who researches cancer risk factors at Brigham and Women's Hospital and Harvard Medical School in Boston.

"This paper confirms what is already known: aspirin use is associated with decreased risk and better survival," said Dr. Gerrit-Jan Liefers, a cancer surgeon at Leiden University Medical Center in The Netherlands.

Liefers said it was interesting that the study found aspirin was associated with smaller tumors but not with whether nearby lymph nodes were involved, which can be an indicator of a cancer's aggressiveness. That's a new finding and will fuel more discussion about how aspirin works, Liefers, who was not involved in the study, told Reuters Health.

"The mechanism is not fully understood," Pawitan said. Some researchers believe the anti-inflammatory and blood thinning effects of aspirin contribute to the lowered risk of certain cancers, he said.

Researchers also aren't sure why aspirin would end up being beneficial for people who develop colon and lung cancer and not for breast or prostate cancer patients, though breast and prostate cancers often have more hormonal factors involved, he said.

It is possible that people who regularly take aspirin tend to have different lifestyles than those who don't, and some other aspect of their lives contributes to their differing cancer risks, Liefers said. The researchers in this study accounted for age, gender and socioeconomic status, but ruling out all other factors in this type of population-based study is always difficult, he said.

Trials that randomly assign people to take aspirin or not will better be able to account for lifestyle differences and are already in the early stages in Asia, The Netherlands and the UK, he said.